The present invention relates to hydrate inhibitors and a method for inhibiting the formation of hydrates in particular to a method for inhibiting the formation of hydrates in the petroleum and natural gas industries.
Hydrates are formed of two components, water and certain gas molecules, such as alkanes of 1-4 carbons, especially methane and ethane, such as those found in natural gas. These `gas` hydrates will form under certain conditions, i.e. when the water is in the presence of the gas and when the conditions of high pressure and low temperature reach respective threshold values. The gas may be in the free state or dissolved in a liquid state, for example, as a liquid hydrocarbon.
The formation of such hydrates can cause problems in the petroleum oil and natural gas industries.
Hydrate formation in the field may cause blocked pipelines, valves and other process equipment.
The problem is particularly of concern as natural gas and gas condensate resources are discovered where operating conditions surpass these threshold values, i.e. in deep cold water and on-shore in colder climates.
Hydrates can also form in association with the underground hydrocarbon reservoir thus impeding production by blockage of reservoir pores.
The problem of hydrate formation is however commonest during gas transportation and processing, the solid hydrate precipitating from moist gas mixtures. This is particularly true with natural gas which when extracted from the well is normally saturated with water. Often in such a case, in a cold climate, hydrates will form in downstream transportation networks and this can cause large pressure drops throughout the system and reduce or stop the flow of natural gas.
Hydrate formation may also occur during natural gas cryogenic liquefaction and separation.
A typical situation where hydrate formation can occur is in offs hore operations where produced fluids are transported in a long vertical pipeline, for example, a riser system. Such produced fluids normally include light gases known to form hydrates and water. In such a situation a temperature of 4.5.degree. C. and a pressure of 150 psi would be sufficient for hydrate formation.
Several methods are known to prevent hydrate formation and subsequent problems in pipelines, valves and other processing equipment.
Physical methods have been used, such as increasing gas temperature in the pipeline, drying the gas before introduction into the pipeline, or lowering the gas pressure in the system. However, these techniques are either expensive or are undesirable because of loss of efficiency and production.
Chemical procedures have also been used. Electrolytes, for example, ammonia, aqueous sodium chloride, brines and aqueous sugar solutions may be added to the system.
Alternatively, the addition of methanol or other polar organic substances, for example, ethylene glycol or other glycols may be used. Methanol injection has been widely used to inhibit hydrate formation. However, it is only effective if a sufficiently high concentration is present since at low concentrations there is the problem of facilitation of hydrate formation. Also for methanol to be used economically under cold environmental conditions there must be early separation and expulsion of free water from the well in order to minimise methanol losses in the water phase.